KB5126646 is a Microsoft update from Patch Tuesday October 2026, released on 13 Oct 2026. According to the Security Update Guide it fixes 892 vulnerabilities (Windows). Affected according to our entries: Microsoft Windows Server 2025, Microsoft Windows Server / Exchange Server. Known issues with this update: 2 with workaround.
After the October 2026 Windows security update, some on-premises Exchange servers might have trouble accessing or sending email. The update introduced a default limit on the memory Endpoint Mapper uses for Endpoint Lookup calls, which can be too low for Exchange under heavy load. Microsoft lists the status as mitigated and is still investigating.
Microsoft · Windows Server 2025
Since the October 13, 2026 updates, Exchange servers running on affected Windows Server versions may become unstable after some time. Microsoft states this is not an Exchange update issue. A registry workaround (EpmapSavedContextCap) is available; no follow-up fix is imminent.
Microsoft · Windows Server / Exchange Server
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